1. Field of the Invention
The present invention relates to a drum-type washing machine, more particularly to a method of reducing tub oscillation that is greater than a predetermined level caused by uneven distribution of laundry in a washing machine having a ball balancer by controlling the movement of the ball balancer in a dehydration operation.
2. Description of the Related Art
In general, a balancer provided in a drum of a washing machine has a hollow ring shape in which a solid-type movable member, such as balls, and oil are provided. When laundry placed in the drum of the washing machine is unevenly positioned and rotated in an unbalanced state, balls of the ball balancer move in a predetermined direction due to a differential centrifugal force caused by the unbalanced rotation, to compensate for the uneven distribution of the laundry unevenly placed in the drum, thereby maintaining the balanced state.
That is, the ball balancer includes a plurality of races, such as inner races and outer races which guide the movement of balls, such that the ball balancer may have different viscosities and ball sizes. In order to prevent resonant oscillation of the tub, that is, to prevent the tub from suddenly oscillating excessively at a predetermined rpm range between about 205 rpm and about 300 rpm due to the differential rotational speed between the drum and the ball before the ball reaches a balancing position, balls provided in at least two races move with a phase difference to compensate for the imbalance caused by the uneven distribution of laundry and balls.
However, the above related art adopts at least two races having different ball sizes and viscosities, so that the manufacturing cost thereof may increase. In addition, the oscillation of the tub greater than the predetermined level is prevented by reducing the occurrence probability of the oscillation greater than the predetermined level without a precise calculation, so that oscillation greater than the predetermined level cannot be perfectly prevented. Further, the related art requires more races in order to further reduce the occurrence probability of the oscillation that is greater than the predetermined level.